In an LED driver circuit it is essential for the LED current to be controlled or kept constant over the expected range of power supply voltage variations and ambient temperature changes. The simplest method to somewhat stabilize the current of an LED would be to use a resistor in series connection with an LED load. But this method has serious drawbacks. In order for the resistor to act more like a current source its value must be large. This would keep the power dissipation on the resistor low but would limit the number of LEDs that can be driven with one resistor. If a low value resistor is used more LEDs can be accommodated but at higher temperatures or supply voltages the power dissipation on the resistor may turn out to be excessive. Hence, with the resistor methodology there is not really current control and at high ambient temperatures or high line voltage conditions the change in the LED load current can shorten the life of the LEDs and can even result in thermal run-away, poor illumination, degradation and sometimes even a total damage to the LEDs.
There is a large number of circuits in prior art designed to stabilize the LED current for power supply voltage and/or ambient temperature variations. These circuits are driven from low AC voltage sources that are rectified with bridge rectifiers before being converted to DC. In order to obtain a pure DC voltage and eliminate rectifier bridge ripple voltage, high quality smoothing capacitors must be used. Such capacitors are bulky, expensive and have a short lifetime as well. Circuits for power factor correction (PFC) are also required. Variable duty cycle control circuits may be included as well most likely for controlling LED illumination by means of pulse width modulation (PWM) techniques that control the LED load current. Also in order for these circuits to provide a constant current for the LED load over a wide DC power supply voltage range, linear regulators or the switched type of regulators like boost or buck converters are used.